1. Field of Invention
The present invention relates to an apparatus which is used on a power line communication network. More particularly, the present invention relates to a transmission schedule constructing apparatus capable of constructing a transmission schedule which is used when a station performs time division multiple access to a power line which is a communication medium.
2. Description of the Related Art
Conventionally, wireless LAN systems and the like have been commercialized as network communication systems in which a signal is transmitted based on a transmission schedule which is previously set by each terminal belonging to a network. For wireless LAN systems, there are standards, such as IEEE802.11b, which utilizes a 2.4-GHz band, IEEE802.11g, which utilizes a 5-GHz band, and the like. These standards are widely used. An attempt is being made to incorporate a concept of Quality of Service (QoS) into an upcoming version (IEEE802.11e) of these standards.
In IEEE802.11 wireless LAN, a control station which is called AP (Access Point) transmits a polling frame to a terminal station which is called STA (STAtion). Thereby, the STA is permitted to transmit a signal. Such a medium access method is called a Point Coordination Function (PCF). With PCF, each STA can perform frame transmission based on a transmission schedule determined by the AP (see IEEE Std 802.11, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications”).
FIG. 15 is a diagram illustrating an example of PCF medium access. An AP and three STAs (STA1 to STA3) constitute a wireless LAN network. The AP transmits a polling frame 201 to give transmission permission to the STA1. After the STAL transmits a data frame 202, the AP transmits a polling frame 203 to the STA2. In response to this, the STA2 transmits a data frame 204. Further, the AP transmits a polling frame 205 to the STA3. In response to this, the STA3 transmits a data frame 206. Thereafter, the same sequence is performed between the AP and each STA, whereby continuous medium access is achieved.
In IEEE802.11e, a medium access method in which the concept of QoS is incorporated into PCF is discussed (see Consumer Communications and Networking Conference, 2004, CCNC 2004, First IEEE, 5-8, Jan. 2004, “A QoS scheduler for IEEE 802.11e WLANs”). In the method discussed in IEEE802.11e, occupation of a transmission medium is achieved by: (1) each STA requesting an AP for a medium occupation time, the STAs and the AP constituting a wireless LAN network; (2) the AP informing each STA of a medium occupation permission time; and (3) the informed STA performing transmission while occupying a medium.
Each STA which constitutes a wireless LAN network informs an AP of information including QoS parameters (TSPEC), such as a data transmission rate, an upper limit of tolerable delay time, and the like, to request an AP for a time for which the STA occupies a transmission medium. FIG. 16 is a diagram illustrating a portion of a list of TSPECs.
When receiving a request for a transmission medium occupation time, the AP schedules a transmission timing based on the requested TSPEC. Based on the transmission schedule thus produced, the AP transmits to each STA a polling frame including medium occupation time information which is called transmission opportunity (TXOP). All STAs other than an STA which has been polled by the AP confirm the TXOP in the polling frame and restrains from transmitting a frame in a time zone indicated by the TXOP. Thereby, each STA constituting an IEEE802.11e wireless LAN can exclusively use a transmission medium while satisfying QoS.
FIG. 17 is a diagram illustrating an exemplary medium access method which is discussed in IEEE802.11e. In a wireless LAN network composed of anAP and three STAs (STA1 to STA3), each STA is assumed to acquire a transmission medium occupation time using the TSPEC of FIG. 16. The AP transmits a polling frame 401 to the STA1. In this case, a time (TXOP) 411 for which the STAL can occupy a medium is written in the polling frame 401. The STA1 transmits a data frame at a time indicated by the TXOP 411 (step S402). When the TXOP 411 is over, the AP designates TXOP and transmits a polling frame 403 to the next STA. In FIG. 17, the AP transmits a polling frame 403 including TXOP 412 to the STA2. This series of steps are repeatedly performed, thereby designating a time for which a medium is occupied by each STA.
Thus, the wireless LAN has a function to perform transmission based on scheduling by the AP. Note that there is a possibility that different transmission schedules may be constructed, depending on a scheduling method in the AP. It is here assumed that scheduling is performed in order of the time at which an allocation request is received.
FIG. 18 and FIG. 19 are diagrams for explaining a scheduling method which is performed in an AP in a wireless LAN. In an example illustrated in FIG. 18, it is assumed that, in a communication network system in which the same transmission schedule is performed in scheduling cycles 511 of 60 msec, the AP performs scheduling based on the following two policies.
The first scheduling policy is that scheduling is performed in order of the time at which an allocation request is received. The AP performs allocation with respect to a data stream in order of the time at which an allocation request is received. When a new allocation request is received, previous allocation is not changed.
The second scheduling policy is that allocation is performed from the head of an unallocated time without a space. Assuming that no allocation has been performed, when receiving a first allocation request, the AP invariably performs allocation from the head of the scheduling cycle 511. Thereafter, when receiving a second allocation request, the AP tries to allocate the earliest time of an unallocated time of the scheduling cycle 511.
It is assumed that the AP receives the following allocation requests. It is also assumed that the AP receives a data stream 501 whose maximum tolerable delay is 20 msec and which requires a time of 5 msec for performing transmission one time, and thereafter, receives a data stream 502 which requires a time of 20 msec in the scheduling cycle 511 and tolerates a maximum transmission delay of 200 msec. FIG. 19 illustrates requirements concerning a transmission time and a delay time of each data stream.
When receiving an allocation request of the data stream 501, the AP determines a time zone to be allocated which satisfies a requested QoS requirement. Specifically, the AP allocates 5 msec ranging from a starting time 521 to a time 522 of the scheduling cycle 511 to the data stream 501 based on the first scheduling policy. Next, the AP allocates 5 msec ranging from a time 524 which is 20 msec after the time 521 to a time 525 to the data stream 501. Thereby, one of the QoS requirements of the data stream 501 that the maximum delay time is 20 msec is satisfied. In addition, the AP allocates 5 msec ranging from a time 526 which is 20 msec after the time 524 to a time 527 to the data stream 501. Thus, allocation is competed which satisfies the QoS requirements requested by the data stream 501.
Further, the AP determines a time zone to be allocated to the data stream 502 in a manner which satisfies the QoS requirements of the data stream 502. The AP allocates to the data stream 502 10 msec from the time 522 at which the first allocation to the data stream 501 is ended in the scheduling cycle 511, so that one of the QoS requirements of the data stream 502 which is an allocation time requirement (10 msec per cycle) is satisfied. Thereby, all scheduling is completed while the QoS requirement of the data stream 502 is satisfied.
However, in a power line communication network in which a power line is used as a communication medium, a problem arises with the above-described scheduling method. Hereinafter, the problem will be described.
The power line communication network is characterized in that the transmission quality of communication medium changes in utility alternating current power source cycles or half cycles thereof. FIG. 20A is a diagram illustrating how the transmission quality changes in synchronization with the utility alternating current power source cycle. FIG. 20B is a diagram illustrating how the transmission quality changes in synchronization with the half cycle of the utility alternating current power source.
However, in conventional scheduling methods, such a change in transmission quality has not been sufficiently taken into consideration. Therefore, for example, the transmission quality may be degraded during data transmission, so that a transmission error occurs and data cannot be normally received. Alternatively, for example, a large volume of data may be transmitted during a time when the transmission quality is poor, so that the data occupies a communication medium for a long time.
The change in transmission quality illustrated in FIG. 20A and FIG. 20B generally has characteristics which vary among transmission and reception stations. FIG. 20C is a diagram illustrating an exemplary fluctuation in transmission quality of different pairs of transmission/reception stations. FIG. 20C illustrates a change 731 in transmission quality between a station A and a station B constituting a power line communication network, and a change 732 in transmission quality between a station C and a station D. Concerning the change 731 in transmission quality, the transmission quality is relatively good in the vicinity of a midpoint of a time zone between a time 741 and a time 742 which corresponds to one power source cycle. In contrast, concerning the change 732 in transmission quality, the transmission quality is relatively good in the vicinity of a head of the time zone between the time 741 and the time 742. Moreover, the change 732 in transmission quality is small compared to the change 731 in transmission quality. Thus, even when transmission quality between transmission and reception stations is taken into consideration, a problem, such as a transmission error or long-time occupation of a communication medium, may occur unless scheduling is performed.
Therefore, an object of the present invention is to provide a transmission schedule constructing apparatus capable of performing scheduling so that data transmission is efficiently and stably, by taking into consideration the transmission quality of a power line communication medium.